1. Field of the Invention
The invention relates to a device for determining a mass flow of a gas flowing in a flow channel.
2. Description of the Related Art
Such a device is known from U.S. Pat. No. 4,754,650. The known device has two ultrasonic transducers that are arranged at the ends of a measuring path and are installed on the wall of a flow channel. With the aid of the ultrasonic transducers, the delay time of sound signals along the measuring path can be determined. From the delay times, the flow rate of the gas flowing in the flow channel and the speed of the sound of the sound signal in the gas can be ascertained. In the case of the known device, the average molecular weight of the gas flowing in the flow channel is determined from the speed of sound ascertained. As a result, it is possible with a known pressure and known temperature to determine the mass flow of the gas flowing in the flow channel.
Furthermore, a built-in unit with an ultrasonic flowmeter, intended in particular for measuring the air mass in a motor-vehicle engine, is known from EP 0 477 419. This built-in unit likewise comprises sound transducers arranged at the ends of a measuring path and with which the delay time of sound signals can be determined. From the delay time of the sound signals, the flow rate of the air flowing in the flow channel can then be determined. By means of an additional temperature sensor and a further pressure sensor, the mass flow of the air flowing in the flow channel can then be ascertained. However, the air taken in always contains a certain proportion of air moisture, and consequently of water mass. On account of the additional water mass, the measured value of the air mass is susceptible to error.